Bastion slide back plates are machined from the highest quality T6 aluminum using the highest tolerances to give you the exact fit you require, then hard coat anodized black and laser engraved, giving you the extreme durability, you deserve! Compatible with Glock models G17 G19 G21 G22 G23 G24 G26 G27 G29 G30 G31 G32 G33 G34 G35 G36 G37 G38 G39 G40 G41, Not compatible with G42 or G43. Great replacement for a plastic factory part. You will love the quality of Bastion products. Many different designs to choose from our stock inventory. We also offer custom engraving!

perhaps I didn't read the product description correctly. I purchased this assuming it was for 8 round Shield mags. Not so. It's designed for 7 round mags.
I did install on a 7 round and found it to be an excellent product.